High power pulsed fiber laser are currently in demand for a number of applications and uses. For example, numerous material processing applications such as memory repair, milling, micro-fabrication, drilling, etc. require pulsed laser systems which provide, among others, the four following characteristics all at the same time and with a great stability over the different conditions of operation and over time:                High pulse energy (50 μJ or higher) with excellent pulse amplitude stability, for processing material at the laser operating wavelength or for efficient frequency conversion;        Excellent beam quality (M2<1.1, astigmatism <10%, beam roundness >95%) with robust single mode operation, for superior processing quality, high throughput processes and efficient frequency conversion;        Narrow linewidth (Δλ<0.5 nm), for small spot sizes and efficient frequency conversion; and        Great flexibility in terms of control of the pulse temporal profile, like pulse to pulse control over the temporal profile at high (>100 kHz) repetition rates.        
In other applications such as remote sensing of different chemical species, the source must additionally provide some level of flexibility over the pulse spectrum.
Scaling the output power without deteriorating other essential characteristics of the laser, such as beam quality or spectral purity, is a main challenge for high power pulsed fiber laser designers. When increasing the pulse peak power, the onset of different nonlinear effects such as Stimulated Brillouin Scattering (SBS), Self-Phase Modulation (SPM), or Stimulated Raman Scattering (SRS) can seriously limit the maximum output power ultimately achievable in a given spectral bandwidth by a pulsed fiber laser system. For narrow linewidth lasers, SBS is generally the first nonlinear effect that manifests when the pulse peak power exceeds a certain level, the so-called SBS threshold. The impacts of SBS are mainly a degradation of the pulse amplitude stability, the appearance of counter-propagating satellite pulses, a roll-off in the laser output power vs pump power curve or even permanent damages to the laser's optical components.
The process of SBS can be described classically as a parametric interaction among the pump wave (which is formed by the optical pulses), the Stokes wave (partially reflected optical pulses) and an acoustic wave. The pump wave generates acoustic waves through electrostriction which in turn causes a periodic modulation of the refractive index in the fiber. This periodic index modulation creates a grating that partially scatters the pump wave through Bragg diffraction, causing the detrimental impacts just described. SBS has been studied extensively since its discovery in 1964. For a general presentation of the SBS theory in the context of optical fibers see for example Govind P. Agrawal, “Nonlinear fiber optics”, Academic Press, San Diego, 2001, chapter 9.
Different SBS mitigation paths exist, such as increasing the fiber mode field diameter to reduce the fluence in the core, thereby increasing the SBS threshold. Such fibers are known to those skilled in the art as Large Mode Area (LMA) fibers. However, this solution has practical limits to the achievable beam quality robustness. Experience has proven that even with sophisticated LMA fiber designs with special index profiles, severe fiber packaging constraints must be carefully addressed to maintain good beam characteristics, even for modest fiber core diameters in the range of 20-30 μm. When such fibers are used for narrow linewidth applications, with pulse durations ranging from 10 ns to 100 ns, the maximum achievable pulse energy seldom exceeds 10 to 15 μJ since it is limited by the onset of SBS even for short lengths of fiber.
Other SBS mitigation paths rely on broadening the SBS gain bandwidth by applying a strain distribution [see J. M. Chavez Boggio, J. D. Marconi and H. L. Fragnito, “8 dB increase of the SBS threshold in an optical fiber by applying a stair ramp strain distribution”, CLEO04 conf. Proceedings, paper CThT30] or a temperature distribution [see J. Hansryd, F. Dross, M. Westlund, “Increase of the SBS Threshold in a Short Highly Nonlinear Fiber by Applying a Temperature Distribution”, J. Lightwave Technol., vol. 19, pp. 1691-1697, November 2001] along the fiber. The strain distribution solution is thought to be more adapted to passive single mode fibers in telecom applications but is not considered practical for high power fiber applications, since applying a controlled strain distribution on a LMA fiber while keeping stable beam characteristics is not really attractive from a practical point of view, due to the quite high modal sensitivity to mechanical constraints (torsion, curvatures, etc.) usually displayed by LMA fibers. In order to use the temperature distribution approach, relatively high temperature gradients (>100° C.) are needed to obtain a valuable SBS threshold increase, which can reduce unacceptably the lifetime and reliability of an LMA fiber incorporating a high index polymer cladding to guide the pump light.
Other known SBS mitigation schemes include designing a fiber with tailored acoustic properties. For example, PAPEN et al. [U.S. Pat. No. 6,587,623] disclose the idea of including an acoustic guiding layer surrounding the fiber core so as to spread the acoustic energy over a large number of acoustic modes, thereby broadening the Brillouin gain spectrum. In another approach HASEGAWA [European patent application no. EP 1 674 901] discloses an acoustic guiding layer specially designed to minimize the overlap between the acoustic modes and the fundamental optical mode. Although attractive for optical fibers having relatively small mode field diameters, the potential of those approaches is again thought to be limited for LMA fibers since the impact of adding the acoustic guiding layer on the fiber optical guiding properties represents a major additional fiber design constraint to obtaining excellent beam characteristics with great robustness. The same argument also applies in general to all approaches implying modifying the fiber structure or its chemistry.
Yet another avenue for addressing SBS related issues is to amplify signals having linewidths significantly broader than the typical SBS gain bandwidth in optical fibers (10-100 MHz). In order to design pulsed fiber laser system producing high peak power pulses having durations in the range of 1 ns to 100 ns, the spectral bandwidth must be broad enough to promote high SBS thresholds, while being narrow enough to enable efficient frequency conversion and avoid problems inherent to less coherent sources. The ideal linewidth is usually in the range of a few GHz to some tens of GHz, a range for which the pulsed laser is considered to be a “narrow linewidth” laser in the context of the different applications mentioned above.
Experience shows that providing a stable pulsed fiber laser with such a spectrum can prove to be difficult. Broader seed sources such as multi-longitudinal mode laser diodes generally exhibits more amplitude noise than narrower sources, due to mode competition, which is detrimental for the pulse amplitude stability. To minimize this amplitude noise the number of longitudinal modes in the seed source must be limited, in which case the spectral width must be of the order of a few hundreds of MHz for acceptable pulse amplitude stability levels to be maintained, clearly well below the ideal range for overcoming SBS.
Alternatively, low-coherence seed sources based on spectrally filtered fluorescence may be used, such as disclosed in international patent application no. WO 2008/086625 (MURISON et al.). Since they do not involve a laser cavity, the fluorescence-based seed sources are not plagued by longitudinal mode beating noise. However, they are relatively inefficient since only a very small fraction of the produced fluorescence (about 0.1% for fiber gain media) is initially selected by the filter element. For polarized sources, the efficiency is even lower as half of the fluorescence power is lost after polarization filtering. Additional optical amplifier stages are therefore often required to boost the output power to usable levels, which increases the overall complexity, component count and cost of the device.
In addition to the practical difficulties listed above, broad linewidth sources also suffer from a susceptibility to nonlinear effects other than SBS, especially SPM, which may quickly broaden the spectrum beyond the maximum acceptable width as the peak power increases in the fiber amplifier. This effect is greater for broad linewidth than for narrow linewidth sources, due mainly to the low coherence and to the important phase noise of the former. This transfers the optical power from the spectral region of interest into large spectral “wings”, thereby reducing the spectral power density of the source. Numerous papers about fiber lasers and amplifiers announcing record peak power levels have been published throughout the years, but often the spectral power density was not discussed or presented, mainly because in reality SPM broadens the spectrum to a point where only a modest fraction of the amplified signal lies in the spectral band of interest. Such a broadening is evidently incompatible with efficient frequency conversion and can create other frequency conversion issues such as poor pulse shape control in the harmonics because of the important frequency chirp developing along the pulse when SPM takes place. Therefore, although SBS is the first nonlinear effect to overcome when scaling the output power of a narrow linewidth fiber laser, it is also very important that the chosen SBS mitigation path does not negatively impact on the mitigation of SPM, which is the next power scaling obstacle.
In another spectral broadening approach, MURISON et al. disclose a seed source based on a frequency chirp induced by amplitude modulation [see International patent application published under no. WO 2008/086625]. In some embodiments, the chirp is obtained using an amplitude modulator having a non-zero chirp parameter. In other embodiments, the injection current of a semiconductor laser diode is modulated in order to generate pulses with a frequency chirp along the pulse. Typically pulses having triangular shapes are generated and an amplitude modulator located downstream the laser diode further gates the pulse in the time domain. The SBS threshold is increased as a result of the spectral broadening corresponding to the frequency chirping. However, one important drawback of the amplitude modulation approach for pulsed lasers is that it induces a strong coupling between the pulse characteristics (amplitude, shape, etc.) and the efficiency of the SBS suppression. The SBS suppression therefore imposes variable limits or constraints on the pulse shape depending upon the conditions of operation (pulse repetition rate, output power, etc.), limiting the flexibility of the device. Another drawback is that the chirp creates an additional pulse shape distortion factor for applications using the laser harmonic wavelengths. As the frequency varies more or less linearly along the pulse, the frequency conversion efficiency will also vary along the pulse, leading to pulse shape distortion. Maintaining stable pulse characteristics from laser to laser and over the laser lifetime becomes usually more difficult to achieve as the number of coupled operating parameters increases.
There remains a need for a pulsed laser system which is able to provide high power pulses suitable for material processing applications or the like.